This invention relates generally to radar navigation systems, and more particularly, to a radar system adapted to accept position information inputs and display current course and waypoint information on a radar display.
The sextant, nautical chart and compass have long been the most important tools of a navigator. With information derived from the sextant, present position may be fixed in latitude and longitude. A desired course may be determined by plotting the present position on the nautical chart together with the position of a desired destination. The compass may then be used to steer along the determined course. Modern navigators have additional electronic devices to aid them. For example, the differences in the time of arrival between a master radio frequency signal and at least two secondary radio frequency signals are used by a Loran to determine the vessel's present position. Some Loran units may also be programmed with the position of the desired destination and thus can also provide an automatic numeric display of the determined course as a magnetic or true bearing. When the destination is far away, it is useful to program the Loran to provide successive courses to intermediate places along the way called waypoints. Marine radars make use of the property that radio frequency signals are reflected by surrounding objects to provide an electronic picture of nearby land masses, vessels and other hazards.
Electronic devices such as Loran and radar have thus been found to be quite helpful. However, when the vessel is underway the navigator's task includes not only considering the determined course but also how to navigate to the destination. Navigation involves not only considering course information from multiple sources such as the Loran's latitude and longitude outputs, the chart, and fixed navigational aids such as buoys, but also visual and radar derived information concerning fixed hazards such as rocks and moving hazards such as other vessels. Thus, successful navigation involves considering information from a variety of sources.
Some Lorans are also adapted to provide a course deviation indicator output, typically a bar graph type display. The navigator can use this display to determine when he has deviated from the determined course. When temporarily avoiding obstructions such as rocks or other vessels, this is of some aid in returning to the determined course. But such indicators can also tend to become yet another source of information to be constantly considered.
One approach to solving the problem of considering navigation information from a variety of sources has been to use an integrated electronic chart display such as that disclosed in U.S. Pat. No. 4,590,569 to Rogoff et al. Such a system provides a single display integrating information available from the radar, Loran, and charts, thus obviating the need to observe multiple information sources. Such an electronic chart device includes a computer having a memory for storing an electronic representation of a chart of the area in which the vessel is presently located. This system also accepts the video output from a radar, and by using the Loran present position output and a highly accurate calibration device, overlays the radar video output on the chart display. True or magnetic bearings to objects of interest may be determined. Present position data from the Loran and other useful information is displayed in numerical form. Such systems tend to be quite a bit more expensive than standard marine radar systems because of the need for accurate calibration, and because chart data must be developed and stored in electronic form. Such systems are quite useful to pilots of large commercial vessels who can justify the added expense. However, a much more common problem is that of operator of a small vessel, such as a fisherman or recreational yachtsman, who find the cost of such electronic map systems to be prohibitive.